Catalytic Hydrogenation of Carbon Dioxide to Formic Acid

24 September 2009

Researchers at the University of Tokyo have developed a new catalyst for the catalytic hydrogenation of carbon dioxide to formic acid with the best reported performance to date. A paper on their work appeared online 23 September in the Journal of the American Chemical Society.

Formic acid is an important chemical product as well as a major player in synthetic chemistry as an acid, a reductant, and a carbon source.

Hydrogenation of carbon dioxide has been widely investigated using transition-metal complexes, especially with rhodium, ruthenium, and iridium catalysts.
Noyori and co-workers carried out the reaction in supercritical carbon dioxide using a Ru(II) catalyst and obtained ammonium formate, alkyl formate, and formamide in high yields. For example, the highest turnover frequency (TOF) (95,000 h-1) has been achieved using RuCl(OAc)(PMe3)4, which is soluble in supercritical CO2. Himeda reported a highly active cationic Cp*Ir(III) catalyst containing phenanthroline derivatives as ligands. The maximum
turnover number (TON) of the hydrogenation reached 222,000 (the
highest TON reported to date) in a basic aqueous solution.

—Tanaka et al.

[The turnover frequency is the number of molecules of a given product made (turnover) per catalytic site per unit time.]

The researchers performed catalytic hydrogenation of carbon dioxide in aqueous potassium hydroxide using a newly synthesized isopropyl-substituted PNP-pincer iridium trihydride complex as a catalyst. Potassium formate was obtained with turnover numbers up to 3,500,000 and a turnover frequency of 150,000 h-1, both of which are the highest values reported to date.

Formic acid is frequently a partially oxidised methane, CH4, molecule, HOCOH (bonding not correctly shown). In this case formic acid is partially reduced CO2. Rather than releasing tons of CO2 in the air we could have piles of formic acid on the ground. CO2 is reduced in plants to form sugars and then cellulose. This cellulose can be carbonised and buried to reduce carbon in the air.

A process that burned coal but only burned the hydrogen in it and left the carbon to be buried in farm soil might still be cheaper than imported oil. ..HG..